Following every stream

An assault on The Hollow with bush-saw and loppers has felled a whole tunnel of 10-foot fuchsia bushes, dense and dark as mangroves, and restored to us the sound of the stream and the little waterfall below the old stone bridge at the road.

To listen to it at full roar in the middle of a wet November night is to count thousands of gallons instead of sheep and to ponder on the great gush of water from the land. Writing this now, looking out to the shore, an arc of browny-grey stretches far beyond the white barrier of surf: a curving stain of peat-silt, carried down by the mountain river.

But what about all the water that doesn't rush into the ocean, or even into lakes and rivers, but that sinks and trickles - even, in places, pours - into the earth, recharging the conduits of groundwater systems we call aquifers? How much of our waste and poison is it taking with it, into fissures and funnels and caverns measureless to man?

The ancient Celts believed in a vast underground lake which not only fed the springs and rivers but was the point of contact with the Otherworld. Ireland, as it happens, has more chance of such subterranean structures than most countries. More than half of the Republic sits upon a great raft of limestone, much of it subject to the destructive process known as karstification.

Just as drumlin and esker were words borrowed from Irish to describe special geological features of a post-glacial landscape, "karst" has emerged from SerboCroat to label eroded limestone uplands in former Yugoslavia - and all other, similar, places, such as the Burren.

Pure limestone can be dissolved into fantastic shapes by the action of weathering and naturally acid water. Just before the Ice Age, much of central Ireland was bare limestone, fretted into pinnacles, gorges and caves like the scenes on some Chinese porcelain (China, too, has its karst country). The glaciers ploughed down a lot of this jagged high relief and then, in their retreat, smothered what was left of the karstic landscape east of the Shannon with a thick layer of boulder clay, sand and gravel.

Further west, notably across mid-Galway, south Mayo and north Clare, the limestone is at or above the surface and riddled with crevices and cavities. The turloughs and swallow-holes, sinking streams and dry valleys, caves and springs, all demonstrate karstification.

Water follows every fissure, enlarges every hole and links one to another in an intricate honeycomb of passages. It wells up into surface floods or falls away into underground rivers and cisterns. It helps feed the flow of surface rivers such as the Brosna, or finds its way into the sea through long tunnels at the coast.

The picture further east, beneath the thick glacial sediments of the midlands, is much more obscure: a buried limestone topography hinted at only by drilling, geophysical surveys, and the bedrock revealed in deep cuttings for roads.

Some very impure limestones are presumed to be non-karstic - less ready to dissolve. In others, ancient conduits in the bedrock are well-sealed with rock and grit. But conduits can unclog: I think of Lough Funshinaun, in Co Roscommon, suddenly disappearing down a plug-hole last year and leaving 16,000 fish to gasp in the mud.

Some karstic systems, though deeply covered, may be slowly accumulating man's modern poisons in the leachate from landfill sites and unofficial dumps. Where water filters slowly through permeable rock, it is often cleaned up on the way. But water in karstified limestone travels fast and far and holds on to its pollutants. More and more wells are being drilled into limestone aquifers, and water-extraction will increase as new industries set up in the countryside.

The urgent need to know more about Ireland's hidden karst-country now preoccupies a special working group of geologists from State, university and private sectors and led by the Geological Survey of Ireland (GSI). Its interest is not only groundwater protection, but the needs of mining, roadbuilding and construction engineers, and local authority planners. One of our serious train derailments - at Tralee, Co Kerry, in 1993 - was caused by subsidence of karstified limestone.

A joint investigation by Trinity College geologists and GSI has built up a remarkable computer database, gathering every recorded clue to local karstification. David Drew of TCD and Donal Daly of GSI have explored and mapped karst landscape that is accessible - the great swathe of limestone from Lough Carra in Co Mayo down to Slieve Aughty and the Burren.

This region is already pierced with hundreds of wells, some of them in the middle of farmyards. Septic tanks with traditional soakage pits are often leaking into the bedrock, together with the run-off from slurry-spreading. E. coli bacteria are widespread in what used to be the purest of spring-water.

For a naturalist, however, much of the fascination of the Drew-Daly report lies in the ebb and flow of underground water. How intriguing to realise that Lough Mask, for example, is really a high point of the local groundwater rather than, as most lakes, a basin collecting the water from surface streams.

Its subterranean conduits to the Corrib rise in the springs of Cong Village, on the other side of the isthmus. The famously futile canal between the two lakes, cut in the mid-19th century, and dry for 90 days a year, is simply by-passed when water is low, but its existence now stops the Cong springs ever reaching their old, fierce flows of more than 50 cubic metres per second.

Divers have been down 20 metres to see the Lough Mask water pouring out from horizontal fissures - enlarged bedding planes in the rock. In a tourist world awash in bottled spring water, Cong should make more fuss about its natural wonders.